The present invention relates generally to apparatus for forming powder materials into granules and for coating granules utilizing a binder liquid and, more particularly, relates to such apparatus of the type utilizing a gaseous fluid for treating the work material during the granulation or coating process.
In various and diverse technologies, it is desirable to process raw material in powder form into larger agglomerated granules and, often, to provide such granules with distinct layers or coats of differing materials. Preferably, such agglomerated granules should be substantially spherical in shape and as uniform as possible in composition and density for ease of flowability and handling of the granules andto enhance the quality of end products formed therewith.
Various forms of granulating apparatus which have been developed over past years have proved in practice to suffer varying disadvantages. So-called spheronizing apparatus utilize a rotating plate disposed within a cylindrical or otherwise round chamber for centrifugally impacting a charge material in moistened or pellet form against the interior wall of the vessel to progressively deform and plasticize the pellets into a spherical shape. This apparatus may also be utilized for applying a powder coating to charge material already processed into spherical granular form. Disadvantageously, this apparatus requires an initial processing of material into pellet form utilizing an extruder and/or a kneading machine and further requires that the spherical granules produced be transferred to a further machine for drying to remove the liquid moistening or wetting agent. Such spheronizing apparatus is difficult, if not impossible, to utilize for producing granules directly from wetted powdered material. Additionally, the resiliency of granules produced by such spheronizing apparatus cannot be selectively varied widely with granules typically being generally dense and hard. The capability of such apparatus for selectively producing a range of granule sizes is also restricted with this apparatus being particularly unsuitable for producing relatively fine granules.
In somewhat similar agitative type granulating apparatus, rotatable stirring blades and disintegrating chopper blades operate in conjunction with one another in a granulating vessel for simultaneously agglomerating and disintegrating powdered material treated with a binder solution to progressively form the powdered material into granules. As with the afore-described spheronizing apparatus, the resiliency of the granules produced by such agitative granulating apparatus cannot be widely varied, the granules characteristically being relatively hard and dense with high compressive strength, and removal of the binder liquid from the granules also requires processing of the granules in a separate drying machine. Moreover, the shape of the granules produced is relatively irregular and is not always spherical, and the granule size distribution is normally wide. This type of granulating apparatus also has no coating capability.
Fluidized bed granulating machines have been developed which essentially combine the functions of an extruder, a spheronizer and a dryer for granule formation and coating. In such apparatus, powdered charge material is fed into a vessel having a stationary screen or perforated plate and is treated on the charge side of the screen or plate with a binder liquid while a gaseous drying fluid such as air is forced through the screen or plate from the opposite side to create a fluidized suspension of the powdered material permitting it to be adhered by the binder liquid into granular form and simultaneously dried. This type of fluidized granulating apparatus is considered disadvantageous in that the granules formed are typically soft and porous with a relatively low density, while the granule shape is normally somewhat irregular and the granule size distribution is relatively wide. While this apparatus is capable of coating a granular charge material with a liquid film, it generally is not possible to perform powder coating operations since the suspension fluid tends to segregate granular and powdered materials due to their differing relative densities. Furthermore, even though film coating operations are possible, any significant irregularity in the granular shapes produced in this type of apparatus makes it difficult to obtain a uniform film coating on such granules.
To overcome some of the aforementioned disadvantages, some fluidized bed granulating and coating apparatus have replaced the stationary screen or plate with a rotating plate which may be either perforated or imperforate. In such apparatus, the rotating plate acts centrifugally on the charge material while the suspension fluid flowing through the spacing between the rotating plate and the vessel, as well as through any perforations in the rotating plate, acts upwardly on the charge material, to simultaneously circulate the material circumferentially about the vessel as well as in a radially spiraling fashion radially outwardly along the rotating plate, axially upwardly along the vessel and radially inwardly and downwardly in return to the rotating plate. Advantageously, this type of fluidized apparatus provides the capability of controlling the density and hardness of the granules produced by varying the velocity of the suspension fluid and the rotational speed of the plate. Additionally, both film coating and powder coating of granules are possible with minimal segregation problems. However, significant problems have been encountered in regulating flow of the suspension fluid through this apparatus to obtain the desired material circulation. As will be understood, the use of an imperforate rotating plate severely restricts the amount of gas flow through the vessel and accordingly perforated plates are preferred to achieve sufficient minimum flow of the suspension fluid. On the other hand, in the use of perforated plates, difficulty is encountered in the proper selection of the size of the perforations in that openings which are too large increase the risk that the powdered charge material may fall downwardly through the perforations while openings which are too small are subject to becoming clogged by the charge material, resulting in increased restriction to the desired flow of the suspension fluid. Accordingly, such apparatus typically are operated only under fluidized conditions utilizing a sufficient flow of the suspension fluid to fully suspend the charge material, thereby to be prevent material loss or clogging of the fluid openings. As a result, however, such apparatus are disadvantageously unsuitable for use under non-fluidized conditions at lesser flow volumes of the suspension fluid.
Additionally, the maximum quantity of charge material which any given apparatus of this type is capable of handling is limited by the combined capacity of the rotating plate and the flow of suspension fluid to maintain the charge material continuously in the desired circulation afore-described. When greater quantities of charge material are attempted to be processed, the limited circulatory capacity of the apparatus results in the creation of a generally stagnant non-moving portion of the charge material above the circulation zone and, in turn, results in relatively wide granule size distribution, irregular granule shape and non-uniform coating of the granules as well as lower productivity per unit volume of the apparatus. Increased rotational speed of the rotating plate merely produces material slippage and resultant frictional heat without enhancing circulation of the charge material and, similarly, increased suspension fluid flow merely exaggerates the stagnation effect and causes the apparatus to operate more in the nature of a stationary screen-type fluidized bed apparatus.
As one means of correcting the problem of controlling the size of the suspension fluid flow openings, some fluidized granulating and coating apparatus have been provided with a modified rotating plate assembly utilizing a plurality of annular rings of increasing diameter arranged in a conical or pyramidal stacked spaced relation to provide annular fluid flow openings between the rings which open radially outwardly rather than upwardly within the vessel to attempt to restrict entry of the charge material into the openings by directing the flow of the suspension fluid radially outwardly of the rotating plate as the fluid passes through the plate. However, since the openings between the rings, albeit radial, are still substantially straight in the radial direction, the plate assembly has been found in practice to be subject nevertheless to the passage of the charge material through the openings and, therefore, such apparatus must still be operated under fluidized conditions only. Moreover, the pyramidal or conical shape of the plate assembly radially outwardly from the centermost annular ring disadvantageously produces a tendency of the radially spiraling circulation of the charge material to be undesirably concentrated at the radially outwardmost extent of the plate assembly. In turn, the flow of the suspension fluid is undesirably restricted through the radially outwardmost annular openings and through the peripheral spacing between the plate assembly and the vessel and, as a result, a greater volume of the suspension fluid flows through the radially inwardmost annular openings and is not fully utilized for circulating and drying the charge material. Accordingly, the radially spiraling circulation of the charge material is impaired and, moreover, the granules produced are unevenly dried by the suspension fluid.